1. Field of the Invention
The present invention is generally in the field of semiconductor packaging. More specifically, the invention is in the field of leadframe packaging.
2. Background Art
A leadframe is used in the fabrication of plastic molded enclosures, also referred to as a xe2x80x9cmolded packagexe2x80x9d in this application. The leadframe is fabricated from a metal such as copper and comprises a paddle which is secured to the body of the leadframe by four tie-bars. The paddle is situated at the center of the leadframe. The leadframe also comprises a number of leads which are secured to the frame.
A die is attached to the top surface of the paddle in a manner known in the art. Respective first ends of respective bond wires are then bonded to respective die bonding pads. Respective second ends of each bond wire are then spanned to respective inner leads in the leadframe and bonded thereto for making the desired electrical connections. In a subsequent stage of the assembly process, the leadframe is placed in a mold cavity in a transfer molding machine and the entire die, bond wires, die bonding pads, and top surface of the paddle are encapsulated in a molding compound.
In an xe2x80x9cexposedxe2x80x9d paddle leadframe based molded package, the bottom of the paddle is not encapsulated in molding compound. The metal on the bottom of the paddle of the leadframe is left exposed in order to attach the bottom of the paddle to the printed circuit board during a re-flow solder process.
The top surface of the paddle is usually silver-plated copper, while the exposed copper bottom of the paddle is usually left unplated. Typical molding compounds have poor adhesion to silver. Therefore, the typical molding compound adheres poorly to the top of the paddle. As a result, most of the protection against separation of the molding compound from the top of the paddle is provided by the four edges along the sides of the paddle. However, the protection provided by the edges of the paddle against separation of the molding compound from the top of the paddle is usually insufficient. Therefore, separation of the molding compound from the top of the paddle may still occur.
It is known in the field of leadframe packaging that poor adhesion of the molding compound to the paddle of the leadframe in an exposed paddle leadframe based molded package is a serious concern. As discussed above, poor adhesion of the molding compound to the silver-plated top of the paddle and insufficient adhesion along the edges of the paddle may result in the separation of the molding compound from the surface of the paddle of the leadframe. This separation is generally referred to as xe2x80x9cdelamination.xe2x80x9d
Delamination can occur during the re-flow process when the bottom of the paddle is soldered to the printed circuit board. The exposed bottom of the paddle is brought into contact with solder on the surface of the printed circuit board. The solder is heated to a temperature of approximately 120 degrees centigrade. At approximately this temperature, the solder becomes molten and the bottom of the paddle of the leadframe is then brought into contact with the molten solder. When the solder cools, a secure connection of the paddle to the printed circuit board will result.
However, there are problems with this process. During the re-flow process, as the temperature rises, a point is reached where the temperature is not yet high enough for the solder to melt, but is high enough to result in an expansion of the copper bottom of the paddle. Because the top of the paddle is encapsulated in molding compound, some support for the paddle in the upward direction is provided. This support on top of the paddle hinders any upward expansion. However, the bottom of the paddle is not encapsulated. As a result, the paddle expands primarily in a downward direction. Because the molding compound has some support from the top and edges of the paddle it tends to stay fixed as the bottom of the paddle pulls downward. This downward expansion of the paddle and the essentially fixed position of the molding compound can lead to a separation of the top surface of the paddle from the molding compound.
One effect of delamination is that the bond wires may be broken. A first end of each bond wire is bonded to a die bonding pad on the die. As discussed above, the die is attached to the top surface of the paddle. The bond wires bonded to the die are encapsulated in the molding compound during the molding process. As stated above, the molding compound tends to stay fixed as the paddle expands downward. Therefore, the bond wires also stay fixed. Thus, in response to the paddle being pulled downward, the bond wires may break. As a result of the problem described above, there are many unusable molded packages in the leadframe assembly process which cause a significant decrease in the yield.
Thus, it is seen that there is a need in the art for a method of providing better adhesion between the molding compound and the top of the paddle in an exposed leadframe based molded package.
The present invention is structure and method for securing a molding compound to a leadframe paddle. According to the invention""s structure and method, improved adhesion between the molding compound and the top surface of the leadframe paddle in an exposed leadframe based molded package is achieved.
According to the invention, half-holes are etched in the top surface of the leadframe paddle. Moreover, according to the invention, each side of the leadframe paddle is provided with a respective anchor. A die is then attached to the top surface of the leadframe paddle. During the molding process, the molding compound covers the die, the anchors, and fills the half-holes in the top surface of the leadframe paddle. The half-holes cause the molding compound to be locked into the paddle. The half-holes also increase the interface surface area between the molding compound and the top surface of the leadframe paddle.
Moreover, during the molding process, the molding compound will encapsulate the anchors provided on each side of the die. When the molding compound hardens around the anchors, the molding compound is reinforced and held in place by the anchors. In essence, the anchors act as supports that extend in four directions within the molding compound to hinder both upward and downward movement of the molding compound. Accordingly, the invention""s half-holes and anchors result in a better adhesion of the molding compound to the top surface of the leadframe paddle.